1. Field of the Invention:
The present invention relates to a fuselage wall inner lining plate that is available for use as an inside wall of the fuselage of an aircraft or the like.
2. Description of the Prior Art:
At first, one example of a fuselage wall structure in the prior art will be described with reference to FIGS. 12 and 13. FIG. 12 is a partial transverse cross-sectional view of a fuselage wall of an aircraft in the prior art taken along a plane perpendicular to a center axis of the aircraft, and FIG. 13 is a partial longitudinal cross-sectional view of the same fuselage wall taken along line Y--Y in FIG. 12. In these figures, reference symbol In designates an inner space of the fuselage, symbol Out designates an outer space of the fuselage, reference numeral 1 designates a fuselage wall inner lining plate having a cylindrical shape, numeral 2 designates a fuselage wall outside plate having a cylindrical shape, numeral 3 designates an annular reinforcement referred to as a "frame", numeral 4 designates another reinforcement extending in the longitudinal direction of the cylindrical fuselage and referred to as a "stringer", numeral 5 designates a damping member for damping vibrations propagating along the outside plate, and numeral 6 designates a sound absorbing material for increasing a sound transmission loss (or simply "transmission loss": hereinafter abbreviated as TL, represented in dB) of the fuselage wall. It is to be noted that the fuselage wall inner lining plate 1 comprises normally isotropic fiber-reinforced plastics or the like.
In a fuselage structure of aircraft in the prior art, the outside plate and the inner lining plate are both substantially cylindrical. The TL of a cylinder close to a ring frequency f.sub.R dependent upon the material and radius of the cylinder is lower compared to when a flat plate made of the same material is employed. FIG. 14 dramatically shows one example of such a comparison, in which at a ring frequency of f.sub.R =350 Hz the TL becomes minimal, and at frequencies close to this ring frequency the TL of a cylinder becomes lower than the TL of a flat plate.
In the case of a passenger plane, since the f.sub.R of an outside plate is normally 300-400 Hz and the f.sub.R of an inner lining plate is about 150-300 Hz, the TL of the fuselage wall consisting of the outside plate and the inner lining plate is, as a whole, within the frequency range of 100-500 Hz. And, in this range, noise on the inside of an aircraft is larger compared to when a flat plate wall having the same cross-sectional structure is employed. On the other hand, in a turboprop engine aircraft, as the principal frequency range of sound generated from a propeller is 200-500 Hz, a lowering of the TL of a cylindrical fuselage wall in these overlapping frequency ranges becomes a main cause of an increase in the sound transmitted to the inside of the aircraft as compared to when a flat plate wall is employed.
In the case where it is contemplated to improve this noise, it is almost impossible to widely change the configuration and material of the outside plate and it is difficult to make any contrivance in the outside plate because of the effect such would have on its being a strength holding member.